Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery-of-choice for general purpose uses such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware whether industrial or military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multicell structure. Each cell contains a plurality of vertical positive and negative plates formed of lead-based alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged contains lead dioxide which is the positive active material and the negative plates contain a negative active material such as sponge lead. This battery has been widely used in the automotive industry for many years and there is substantial experience and tooling in place for manufacturing this battery and its components and the battery is based on readily available materials, is inexpensive to manufacture and is widely accepted by consumers.
However, during discharge, the lead dioxide (a fairly good conductor) in the positive plate is converted to lead sulfate, an insulator. The lead sulfate can form an impervious layer encapsulating the lead dioxide particles which limits the utilization to less than 50 percent of capacity, typically around 30 percent. The power output is significantly influenced by the state-of-discharge of the battery, since the lead sulfate provides a circuit resistance whenever the battery is under load. Furthermore, the lead sulfate can grow into large, hard, angular crystals, disrupting the layer of paste on the grid resulting in flaking and shedding of active material from the grid. Power consumption during charge is also increased due to the presence of the lead sulfate insulator. The lead sulfate crystals in the negative electrode can grow to a large, hard condition and, due to their insulating characteristics, are difficult to reduce to lead. Even when very thin pastes are utilized, the coating of insulating lead sulfate interferes with power output. Thus, power capability is greatly influenced by the state-of-charge of the battery.
The positive plate of the lead-acid battery is the plate that normally fails in a deep cycle application. As a battery is cycled, the positive paste softens and eventually causes the battery to fail. Failure can occur in a number of ways. As the paste softens, it can lose contact with the plate and become inactive. This reduces the capacity of the battery and eventually leads to battery failure. If the softened active material falls to the bottom of the battery and bridges the gap between a positive and negative plate, the battery will fail from short circuiting.
The softening of the active material also exposes the grid to more sulphuric acid. This accelerates grid corrosion and can produce an insulating layer on the grid which prevents the active material from being in good electrical contact with the grid. In this case, the battery would fail as a result of an interface problem between the grid and active material. Grid corrosion also produces grid growth which separates the grid from the positive active material. In this case, the battery will lose capacity and eventually fail. The major problem associated with extending the life of lead-acid batteries is maintaining the integrity of the positive plate while it is cycled.
Another problem associated with lead-acid batteries is that the electrical conductivity for a discharged or sulphated plate is very low. Discharged portions of the plate can act to electrically isolate and prevent other portions of the plate from either charging or discharging. The utilization of the plate's active material during a discharge is reduced as a result of this electrical isolation.
An apparent solution to this problem would be the addition of a conductive filler to the paste. The filler must be thermodynamically stable to the electrochemical environment of the cell, both with respect to oxidation and reduction at the potential experienced during charge and discharge of the cell, and to attack by the acid. It has been attempted to increase the conductivity of the paste by adding a conductive filler such as graphite.
Graphite has been used successfully as a conductive filler in other electrochemical cells, such as in the manganese dioxide positive active paste of the common carbon-zinc cell, and mixed with the sulfur in sodium-sulfur cells. However, even though graphite is usually a fairly inert material, it is oxidized in the aggressive electrochemical environment of the lead-acid cell to acetic acid. The acetate ions combine with the lead ion to form lead acetate, a weak salt readily soluble in the sulfuric acid electrolyte. This reaction depletes the active material from the paste and ties up the lead as a salt which does not contribute to production of storage of electricity. Highly conductive metals such as copper or silver are not capable of withstanding the high potential and strong acid environment present at the positive plate of a lead-acid battery. A few electrochemically-inert metals such as platinum are reasonably stable. But the scarcity and high cost of such metals prevents their use in high volume commercial applications such as the lead-acid battery. Platinum would be a poor choice even if it could be afforded, because of its low gassing over-potentials.
Hughel (U.S. Pat. No. 3,466,197) discloses the addition of 5-25 percent by weight of lead fibers to the positive paste of a deep-cycle lead-acid battery. Hughel also added 0.1 to 1 percent by weight of non-conductive polymeric fibers to increase the strength of the plates. The presence of non-conductive fibers increases bulk and weight and reduces efficiency of the plates. Furthermore, lead fibers are subject to significant stress corrosion during charge-recharge cycling. Pure lead fibers contain microcracks. Stress corrosion starts at the microcrack and continues until the fiber is consumed and loses its reinforcement function. It is very difficult to manufacture pure lead fibers without microcracking. Hughel suggests strengthening a conductive latticework by use of a tissue of lead coated glass fibers. However, no lead coated wire existed with the requisite reinforcement coating and adequate strength.
Rowlette (U.S. Pat. No. 4,507,372) discloses adding SnO.sub.2 coated glass fibers to a positive paste to maintain conductivity during charge and discharge. Again, there is an increase in bulk and loss of capacity since lead oxide is displaced with the tin oxide coated glass.